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The capillary pyrites occurs in delicate capillary crystals, grouped, parallel, diverging or interwoven, slightly flexible, having a metallic lustre, and a colour passing from yellow to steel-grey. There is, lastly, the hepatic pyrites, so named from the liver-brown colour which it assumes from exposure to the air. In the fresh fracture its colour is pale brass-yellow, inclining to steel-grey. It occurs massive, of various imitative forms, and crystallized in six-sided prisms, or six-sided pyramids it has less lustre than the others, and is more subject to decomposition. What has been named magnetic pyrites, distinguished as the name implies, by its magnetic quality, of which the others are destitute, has been considered as forming a distinct species. Its colour is deeper, being intermediate between brass-yellow and copper-red, and approaching even to brown, often tarnished its lustre is also inferior, but is still metallic. It occurs only massive or disseminated. Its fracture is compact: it is hard and brittle: its specific gravity is 4.5. It appears from Mr. Hatchet's analysis of it to differ from the other iron pyrites, in containing a larger proportion of metal, to which, no doubt, its quality of being attracted by the magnet is owing.

PYROLA, in botany, winter-green, a genus of the Decandria Monogynia class and order. Natural order of Bicornes. Ericæ, Jussieu. Essential character: calyx five-parted; petals five ; capsule superior, five-celled, opening at the corners, many-seeded; anthers with two pores. There are six species, natives of the north of Europe.

PYROLIGNOUS and PrROTARTAROUS acids. When wood is distilled in close vessels, it always yields more or less of an acid juice: the same remark applies to the salt called tartar. These liquids were distinguished by the name of pyrolignous and pyrotartarous acids: but they are now known to be only the acetic, disguised by the presence of a peculiar oil.

PYROMETER, an instrument for measuring the expansion of bodies by heat. The whole art in forming an instrument, adapted to this purpose, is, so as to ren der it capable of showing very small expansions of solid bodies. Different instruments have been invented for this purpose; of the greater number of which it is scarcely necessary to give a detailed account. The difficulty of contriving an unexceptionable instrument of this kind has arisen partly from the difficulty of finding a substance not liable to be altered by a high temperature, and which shall suffer a change of volume sufficiently perceptible to be accurately measured, and partly from that of finding a measure, which shall not itself be affected by the high temperature, and be at the same time sufficiently delicate.

The pyrometer, in which, perhaps, these difficulties have been most effectually surmounted, and which has come into most general use, is that invented by the late Mr. Wedgewood. The pure earth, named alumina, and the different earths (the clays) in which it predomi nates, have the singular property of not expanding, but of contracting, by heat. This contraction begins to become evident, when the clay is raised to a red heat; it continues to proceed until it vitrifies, and the total contraction, in pure clays, exceeds considerably one-fourth part of the volume in every direction. It occurred to Mr. Wedgewood, that from this property it might be employed in the construction of a pyrometer. The contraction that the clay suffers is permanent, or it does not return to its former dimensions when cold. The degree of contraction it has suffered, therefore, can be ascertained without any source of fallacy, and will indicate the extreme of temperature to which it has been exposed.

This pyrometer consists of a gauge, composed of two straight pieces of brass, twenty-four inches long, divided into inches and tenths, and fixed in a brass plate so as to converge; the space between them, at the one extremity, being five-tenths of an inch, and at the other three-tenths. The pyrometrical pieces of clay are small cylinders, flattened on one side, made in a mould, so as to be adapted exactly to the wider end. It is evident that, in exposing one of these pieces to a high temperature, the contraction it has suffered may be measured, by the length to which it can be slid into the converging groove or gauge.

The utility of this instrument, it was obvious, would be much increased by connecting it with the mercurial thermometer, and by ascertaining the proportion between the degrees of each; and this was done by Mr. Wedgewood. The scale of his pyrometer commences at red-heat fully visible in day-light. The mercurial thermometer cannot easily measure any temperature above 500° or 550°; and hence, between the termination of the scale in the one, and its commencement in the other, there is a range of temperature requiring to be measured. This Mr. Wedgewood did, by the expansions of a square piece of silver, measured in a guage of earthen-ware, constructed in the same way as his pyrometer; and by the same method, he found out the proportion between each degree of his scale, and that of any of the usual thermometrical_scales. Each degree of his pyrometer he found to be equal to 130° of Fahrenheit. The commencement of his scale, or the point marked 0, corresponds with 10773° of Fahrenheit's scale. From these data, it is easy to reduce either to the other, through their whole range. The scale of Wedgewood includes an extent of temperature equal to about 32,000 of Fahrenheit, or 54 times as much as that between the freezing and boiling points of mercury. Its commencement, as has been stated, is at 10773° of Fahrenheit, or red-heat fully visible in day-light; its extremity is 240°; but the highest heat that he measured with it is 160°, or 21,877° of Fahrenheit; being the temperature of a small airfurnace, and 30° degrees of his scale above the point at which cast iron melts. Guyton has proposed a pyrometer for measuring high temperatures, in which platina, a metal not fusible even at very intense heats, is employed as the measure of expansion. A rod or plate of this metal is placed horizontally in a groove formed in a mass of hardened white clay; one extremity of the rod is supported on the mass which terminates the groove; the other presses against a bended lever of platina, the longest arm

of which forms an index to a graduated arc. The expansion, which the rod of metal suffers from exposure to heat, is indicated by the change of position in this index. The mass of clay, being highly baked, will not introduce any important error from its contraction; and the expansion, which it may suffer during the exposure to heat, will affect only the small distance between the axis of motion of the index and the point of contact of the plate, so as rather to diminish the effect than to increase it. Platina, having the important advantage of not melting by any heat we have to measure, and of not suffering any chemical change from it, is well adapted to the construction of a pyrometer.

Besides these, various metallic pyrometers have been invented, capable of measuring low temperatures, by the expansion being multiplied by the aid of wheels, levers, or other mechanical contrivances, or being magnified by microscopes. Such are the pyrometers of Muschenbroeck; that described by Ferguson; one invented by Mr. Ellicot, with which he measured the expansions of various metals; one by Mr. Smeaton, and applied to the same purpose; Mr. Ramsden's, superior to the preceding ones in delicacy and accuracy; Mr. Crichton's, in which advantage is taken of the dif ference of expansion between a rod of zinc and a rod of iron, to give a curvature to the bar composed of the united rods, proportioned to the temperature to which they are raised; by which bending motion is given to an index, that at its other extremity, where the scale is marked, describes a considerable space; and, lastly, one by Regnier, on a principle somewhat similar, of which a report is presented to the French National Institute. The strict accuracy of these instruments may, from the nature of their construction, be regarded as doubtful. It has been found, by Ellicot's pyrometer, that the expansion of bars of different metals, by the same degree of heat, is as follows:

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It is found in many parts of Germany; also in Fifeshire in Scotland, in the sand of the sea-shore. It is employed in almost every kind of jewellery, and generally set in gold foil. The very small grains are powdered, and used in the stead of emery in cutting softer stones. This was for merly considered as a variety of the garnet, and denominated the Bohemian Garnet, from its occurring in that country in great beauty and perfection. Werner has given the title to a distinct species, on account of its colour, transparency, and want of crystallization.

PYROPHORUS, in chemistry, a compound substance, which takes fire on the admission of the atmospheric air. It is prepared by exposing to heat in an iron pot, three parts of alum, with one part of flour; the mixture liquefies, and is to be stirred constantly till the whole becomes grey, and easily reducible to powder while hot.

The coarse powder is put into a coated phial, so as nearly to fill it; the mouth of the phial is stopped with a small plug of clay, and is placed in a crucible, and surrounded with sand up to the neck. The crucible is heated to redness, until a blue flame appears at the mouth of the phial; when this has continued ten minutes, the crucible is removed from the fire, and the phial, when sufficiently cold, is accurately stopped. This substance inflames in atmospheric air; in a moist atmosphere, the inflammation is much more speedy, and in a dry air it can scarcely take place. It burns also very brilliantly in oxygen gas, in nitrous gas, and in oxymuriatic acid gas; and is inflamed by the sulphuric and nitric acids. See ALUM.

PYROSTRIA, in botany, a genus of the

Tetrandria Morogynia class and order. Natural order of Rubiaceæ, Jussieu. Generic character: calyx very small, fourtoothed; corolla bell-shaped, five-cleft, tomentose in the throat; stamina four; pistils one; stigma capitate; pericarpium drupe, pear-shaped, inferior, small, eightstreaked; nuts eight, one-seeded. There is but one species, viz. P. salicifolia, a native of the island of Mauritius.

PYROTECHNY, is, properly speaking, the science which teaches the management and application of fire in divers operations: but, in a more limited sense, and as it is commonly used, it refers chiefly to the composition, structure, and use of artificial fire-works. The ingredients are, 1. saltpetre, purified for the purpose: 2. sulphur: and 3. charcoal. Gunpowder is likewise used in the composition of fire-works, being first ground, or, as it is technically termed, mealed. Camphor and gum-benzoin are employed as ingredients in odoriferous fire-works. The proportions of the materials differ very much in different fire-works, and the utmost care and precaution are necessary in the working them to a state fit for use, and then in the mixing. In this work we cannot enter on the subject with a sufficient degree of minuteness to teach the method of making of fire-works, and shall therefore content ourselves with a brief notice of the proportions of the materials in some of the more common and more interesting articles in use.

The charges for sky-rockets are made of saltpetre, four pounds; brimstone one pound; and charcoal one pound and a half; or by another direction, saltpetre, four pounds; brimstone one pound and a half; charcoal, twelve ounces; and mealed powder two ounces. These proportions vary again according to the size of the rocket; in rockets of four ounces, mealed powder, saltpetre, and charcoal, are used in the proportions of 10: 2 and 1; but in very large rockets the proportions are saltpetre, four; mealed powder and sulphur one each. When stars are wanted, camphor, alcohol, antimony, and other ingredients are required, according as the stars are to be blue, white, &c. In some cases gold and silver rain is required; then brass-dust, steel-dust, sawdust, &c. enter into the composition; hence the varieties may be almost indefinite. With respect to colour, sulphur gives a blue, camphor a white or pale colour, saltpetre a clear white yellow, sal ammoniac a green, antimony a reddish, rosin a copper colour.

These ma

terials require preparation before they are fit for use; and before a person can be qualified for the business of fire-work making, he must understand the method of making the moulds, cases, &c. and be acquainted with the instruments used in the art, their dimensions and materials. To discuss the several topics connected with the business would require a space very much larger than could be afforded in this work we shall therefore content ourselves with this notice, referring our readers to distinct treatises on the subject, which are to be found in the English and French languages.

PYRUS, in botany, a genus of the Icosandria Pentagynia class and order. Natural order of Pomaceæ. Rosacea, Jussieu. Essential character: calyx five-cleft; petals five; pome inferior, five-celled, many-seeded. There are thirteen species, with many varieties. The P. communis, common pear-tree, grows to a lofty height, with upright branches, the twigs or branchlets hanging down; leaves ellip. tical, obtuse, serrate; the younger ones clothed with a deciduous cotton underneath and along the edge; stipules linear; flowers in terminating villose corymbs; corolla snow-white; pome produced at the base; hard and acerb, in the wild state, with five cells in the middle, each two-valved, containing two seeds. The wood of the pear is light, smooth, and compact; it is used by turners, also for joiners' tools, for picture frames to be stained black; the leaves afford a yellow dye, and may be used to give a green to blued cloths: the juice of the fruit fermented is called Perry. The P. malus, common apple tree, is very spreading, with the branches and twigs irregular and twisting, more horizontal than in the pear; leaves ovate, serrate, the younger ones pubescent underneath; stipules linear; flowers in terminating sessile, vil-lose umbels; corollas white inside, and finely tinged with red on the outside; fruit roundish, umbilicate at the base, acid. The wood of the wild apple is tolerably hard, it turns very clean, and when made into cogs for wheels acquires a polish, and lasts a long time; the bark affords a yellow dye: the acid juice of the fruit is called verjuice; it is much used in recent sprains, and in other cases as an astringent or repellent. For a full description of the numerous varieties of pears and apples, the reader is referred to Martyn's edition of Miller's "Gardener's and Botanist's Dictionary." PYTHAGORAS, in biography, one of

the greatest philosophers of antiquity, was born about the forty-seventh Olym piad, or 590 years before Christ His father's principal residence was at Samos; but being a travelling merchant, his son Pythagoras was born at Sidon, in Syria; but soon returning home, our philosopher was brought up at Samos, where he was educated in a manner that was answerable to the great hopes that were conceived of him. He was called "the youth with a fine head of hair;" and from the great qualities that soon appeared in him, he was regarded as a good genius sent into the world for the benefit of mankind

Samos, however, afforded no philosophers capable of satisfying his thirst for knowledge; and therefore, at eighteen years of age, he resolved to travel in quest of them elsewhere. The fame of Pherecydes drew him first to the island of Syros; from hence he went to Miletus, where he conversed with Thales. He then travelled to Phoenicia, and stayed some time at Sidon, the place of his birth; and from hence he passed into Egypt, where Thales and Solon had been before him.

Having spent twenty-five years in Egypt to acquire all the learning and knowledge he could procure in that country, with the same view he travelled through Chaldea, and visited Babylon. Returning after some time, he went to Crete; and from hence to Sparta, to be instructed in the laws of Minos and Lycurgus. He then returned to Samos; which finding under the tyranny of Polycrates, he quitted again, and visited the several countries of Greece. Passing through Peloponnesus, he stopped at Pholius, where Leo then reigned; and in his conversation with that prince, he spoke with so much eloquence and wisdom, that Leo was at once ravished and surprised.

From Peloponnesus he went into Italy, and passed some time at Heraclea and at Tarentum; but made his chief residence at Croton; where, after reforming the manners of the citizens by preaching, and establishing the city by wise and prudent councils, he opened a school, to display the treasures of wisdom and learning he possessed. It is not to be wondered that he was soon attended by a crowd of disciples, who repaired to him from different parts of Greece and Italy.

He gave his scholars the rules of the Egyptian priests, and made them pass through the austerities which he himself

had endured. He at first enjoined them a five years' silence in the school, during which they were only to hear; after which leave was given them to start questions, and to propose doubts, under the caution, however, to say, "not a little in many words, but much in a few." Having gone through their probation, they were obliged, before they were admitted, to bring all their fortune into the common stock, which was managed by persons chosen on purpose, and called economists, and the whole community had all things in common.

The necessity of concealing their mysteries induced the Egyptians to make use of three sorts of styles, or ways of expressing their thoughts; the simple, the hieroglyphical, and the symbolical. In the simple, they spoke plainly and intelligibly, as in common conversation; in the hieroglyphical, they concealed their thoughts under certain images and characters; and in the symbolical, they explained them by short expressions, which, under a sense plain and simple, included another wholly figurative. Pythagoras borrowed these three different ways from the Egyptians in all the instructions he gave; but chiefly imitated the symbolical style, which he thought very proper to inculcate the greatest and most important truths; for a symbol, by its double sense, the proper and the figurative, teaches two things at once; and nothing pleases the mind more than the double image it represents to our view. In this manner Pythagoras delivered many excellent things concerning God and the human soul, and a great variety of precepts, relating to the conduct of life, political as well as civil; he made also some considerable discoveries and advances in the arts and sciences. Thus, among the works ascribed to him, there are not only books of physic and books of morality, like that contained in what are called his "Golden Verses," but treatises on politics and theology. All these works are lost; but the vastness of his mind appears from the wonderful things he performed. He delivered, as antiquity relates, several cities of Italy and Sicily from the yoke of slavery; he appeased seditions in others; and he softened the manners, and brought to temper the most savage and unruly spirits of several people and tyrants. Phalaris, the tyrant of Sicily, it is said, was the only one who could withstand the remonstrances of Pythagoras; and he, it seems, was so enraged at his discourses, that he ordered him to be put to VOL. X.

death. But though the lectures of the philosopher could make no impression on the tyrant, yet they were sufficient to reanimate the Sicilians, and to put them upon a bold action. In short, Phalaris was killed the same day that he had fixed for the death of the philosopher.

Pythagoras had a great veneration for marriage; and therefore himself married, at Croton, a daughter of one of the chief men of that city, by whom he had two sons and a daughter. One of the sons succeeded his father in the school, and became the master of Empedocles. The daughter, named Damo, was distinguished both by her learning and her virtues, and wrote an excellent commentary upon Homer. It is related, that Pythagoras had given her some of his writings, with express commands not to impart them to any but those of his own family; to which Damo was so scrupulously obedient, that even when she was reduced to extreme poverty, she refused a great sum of money for them.

From the country in which Pythagoras thus settled and gave his instructions, his society of disciples was called the Italic sect of philosophers, and their reputation continued for some ages afterwards, when the Academy and the Lyceum united to obscure and swallow up the Italic sect.

Pythagoras's disciples regarded the words of their master as the oracles of a god; his authority alone, though unsupported by reason, passed with them for reason itself; they looked upon him as the most perfect image of God among men. His house was called the temple of Ceres, and his court-yard the temple of the Muses: and when he went into towns, it was said he went thither, "not to teach men, but to heal them."

Pythagoras was persecuted by bad men in the last years of his life, and some say he was killed in a tumult raised by them against him; but, according to others, he died a natural death, at 90 years of age, about 497 years before Christ.

Beside the high respect and veneration the world has always had for Pythagoras, on account of the excellence of his wisdom, his morality, his theology, and politics, he was renowned as learned in all the sciences, and a considerable inventor of many things in them; as arithmetic, geometry, astronomy, music, &c. In arithmetic, the common multiplication table is, to this day, still called Pythagoras's table. In geometry, it is said he invented many theorems, particularly these

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